Temperature nonionic microemulsions

Schomacker compared the use of nonionic microemulsions with phase transfer catalysis for several different types of organic reactions and concluded that the former was more laborious since the pseudo-ternary phase diagram of the system had to be determined and the reaction temperature needed to be carefully monitored [13,29]. The main advantage of the microemulsion route for industrial use is related to the ecotoxicity of the effluent. Whereas nonionic surfactants are considered relatively harmless, quaternary ammonium compounds exhibit considerable fish toxicity. 

Differential scanning calorimetry (DSC) was used to determine the kinetics of polymerization and the glass transition temperature of the solid polymer. Preliminary results indicate the dependence of kinetics on the microstructure as determined using Borchardt and Daniels method (26). The reaction order, rate constant, and conversion were observed to be dependent on the initial microstructure of the microemulsions. The apparent glass transition temperature (Tg) of polystyrene obtained from anionic surfactant (SDS) microemulsions is significantly higher than the Tg of normal bulk polystyrene. In contrast, polymers from nonionic microemulsions show a decrease in Tg. Some representative values of Tg are shown in Table I. 

With nonionic surfactants, both types of microemulsions can be formed, depending on the conditions. With such systems, temperature is the most cracial factor as the solubility of surfactant in water or oil is temperature-dependent. Microemulsions prepared using nonionic surfactants will have a limited temperature range. 

Figure 25 illustrates with the case of nonionic microemulsions at low temperatures that relaxation is constant up to high volume fractions for the case of constant-size microemulsions. Here, AR is plotted as a function of the droplet volume fraction O. At 25°C the droplet size and shape are independent of O, and AR is constant. At higher temperatures (29 and 31 °C) the droplet size increases with increasing O as shown by the increase in AR. 

Qutubuddin and coworkers [43,44] were the first to report on the preparation of solid porous materials by polymerization of styrene in Winsor I, II, and III microemulsions stabilized by an anionic surfactant (SDS) and 2-pentanol or by nonionic surfactants. The porosity of materials obtained in the middle phase was greater than that obtained with either oil-continuous or water-continuous microemulsions. This is related to the structure of middle-phase microemulsions, which consist of oily and aqueous bicontinuous interconnected domains. A major difficulty encountered during the thermal polymerization was phase separation. A solid, opaque polymer was obtained in the middle with excess phases at the top (essentially 2-pentanol) and bottom (94% water). The nature of the surfactant had a profound effect on the mechanical properties of polymers. The polymers formed from nonionic microemulsions were ductile and nonconductive and exhibited a glass transition temperature lower than that of normal polystyrene. The polymers formed from anionic microemulsions were brittle and conductive and exhibited a higher Tj,. This was attributed to strong ionic interactions between polystyrene and SDS. 

Garti, N.,Anserin, A., Ezrahi, S., Tiunova, L, and Berkovic, B. 1996. Water behaviour in nonionic surfactant systems I Subzero temperature behavior of water in nonionic microemulsions studied by DSC. J. Colloid Interface Sci., 178, 60-68. 

One particular advantage of using mixtures of nonionic and ionic surfactants as microemulsifiers is the formation of temperature-insensitive microemulsions (17). Recall that the temperature-dependence of the phase behaviour of balanced microemulsion mixtures with ionic surfactants such as Aerosol OT (see Figure 4.10) is opposite to that found for ethoxylated alcohols (see Figure 4.5). Upon raising the temperature of Aerosol OT mixtures, a hydrophilic shift occurs (2-3-2), although with hoxylated alcohols, a lipophilic shift occurs (2-3-2). Intuitively, upon mixing ionic and nonionic surfactants, the temperature dependence should cancel at a particular ratio (5) of the two surfactants. 

Measurements of the optimal temperature for microemulsion formation (T) are plotted as a function of AOT surfactant concentration 0) and salinity ( ) in Figure 4.11 (46). Rather than T simply averaging upon mixing the two surfactants, due to the opposing temperature-dependence of the nonionic and ionic systems, the curves of constant salinity (e) diverge at a value of 8 70 wt%. A pole is found in the phase behaviour (a flip in the curvature of the lines of constant e) for e between 0.8 and 1.0 wt%. Thus, at <5 ... 

Figure 4.11. Phase behaviour of mixtures of ionic and nonionic surfactants in microemulsion systems of C12E5/AOT/ decane/water/NaCl. Optimal temperatures for microemulsion formation (T) are plotted as a function of wt% AOT in the surfactant mixture (5), and wt% NaCl in water (e). Reproduced by permission of the American Chemical Society (from Kahlweit and Strey (46))...

Nonionic surfactants with sugar hydrophilic groups -alkylpolyglucoside surfactants - are highly hydro-philic, and form temperature-insensitive microemulsions upon addition of alcohol (47, 48), as do sucrose ester surfactants (49). Zwitterionic surfactants such as lecithins also form microemulsions upon the addition of cosurfactant (50-52). In addition, trisiloxane surfactants microemulsify silicon oils (53), and fluorocarbon-tailed surfactants microemulsify fluorinated oils (54, 55). 

Precise determination of phase boundaries allows us to demonstrate an excess of solubilization in a nonionic microemulsion and to quantify the curvature variation with the amount of lindane adsorbed in the surfactant film. We find that stable microemulsions exist with a larger amount of lindane relative to the oil than the saturation value in the same oil alone at the same temperature. This excess is quantified by titration in the Winsor III domain with the assumption of Leodidis and Hatton i.e. the composition of excess oil (water) is equal to the... 

This provides for interesting applications because, in contrast to fatty alcohol ethoxylates, temperature-stable microemulsions can be formed with alkyl polyglycosides. By varying the surfactant content, the type of surfactant used, and the oil/water ratio, microemulsions can be produced with custom-made performance properties, such as transparency, viscosity, refatting effect, and foaming behavior. In mixed systems of, say, alkyl ether sulfates and nonionic coemulsifiers (alkyl polyglycoside), extended microemulsion areas... 

The previous stability discussion does not apply to microemulsions because they are thermodynamically stable isotropic solutions. Microemulsions differ from macroemulsions in that their oil-surfactant components can exist as cylindrical or bicontinuous strucmres in addition to spheroids (9). The stability and structure of nonionic microemulsions is governed by temperature. As the temperature is increased to the phase inversion temperature (PIT), the structure converts to bicontinuous and then it inverts as the PIT is exceeded. Similar effects can be accomplished for ionic microemulsions by adding electrolyte and certain cosurfactants. Microemulsions may also lose stability when diluted. 

Temperature Dependent Interactions in Nonionic Microemulsions — Bending Energy Contribution... 

Many solutions of common nonionic surfactants and water separate into two phases when heated above a certain temperature (the cloud point), and some investigators call the phase of greater surfactant concentration, a microemulsion. Thus, there is not even universal agreement that a microemulsion must contain oil. 

Results described in the literature have resulted in several patents, such as one for the improvement of the transport of viscous crude oil by microemulsions based on ether carboxylates [195], or combination with ether sulfate and nonionics [196], or several anionics, amphoterics, and nonionics [197] increased oil recovery with ether carboxylates and ethersulfonates [198] increased inversion temperature of the emulsion above the reservoir temperature by ether carboxylates [199], or systems based on ether carboxylate and sulfonate [200] or polyglucosylsorbitol fatty acid ester [201] and eventually cosolvents which are not susceptible for temperature changes. Ether carboxylates also show an improvement when used in a C02 drive process [202] or at recovery by steam flooding [203]. 

The phase inversion temperature (PIT) method is helpful when ethoxylated nonionic surfactants are used to obtain an oil-and-water emulsion. Heating the emulsion inverts it to a water-and-oil emulsion at a critical temperature. When the droplet size and interfacial tension reach a minimum, and upon cooling while stirring, it turns to a stable oil-and-water microemulsion form. " ... 

The packing ratio also explains the nature of microemulsion formed by using nonionic surfactants. If v/a 1 increases with increase of temperature (as a result of reduction of a ), one would expect the solubilisation of hydrocarbons in nonionic surfactact to increase with temperature as observed, until v/a l reaches the value of 1 where phase inversion would be expected. At higher temperatures, va l > 1 and water in oil microemulsions would be expected and the solubilisation of water would decrease as the temperature rises again as expected. 

We saw in Section 8.6 that phase diagrams are an effective way of representing the complex behavior of surfactant systems. Let us take a look at microemulsions in terms of phase diagrams. It turns out that nonionic surfactants form microemulsions at certain temperatures without requiring cosurfactants. Since only three components are present, these have somewhat simpler phase diagrams this kind of system offers a convenient place to begin. 

What are the most important factors influencing the type of microemulsion Here again we have to distinguish between nonionic and ionic surfactants. For nonionic surfactants, often alkylethylene glycols, temperature is the dominating parameter for the structure of a microemulsion. For ionic surfactants, mostly SDS or CTAB, the salt concentration dominates... 

The ultralow interfacial tension can be produced by using a combination of two surfactants, one predominantly water soluble (such as sodium dodecyl sulfate) and the other predominantly oil soluble (such as a medium-chain alcohol, e.g., pentanol or hexanol). In some cases, one surfactant may be sufficient to produce the microemulsion, e.g., Aerosol OT (dioctyl sulfosuccinate), which can produce a W/O microemulsions. Nonionic surfactants, such as alcohol ethoxylates, can also produce O/W microemulsions, within a narrow temperature range. As the temperature of the system increases, the interfacial tension decreases, reaching a very low value near the phase inversion temperature. At such temperatures, an O/W microemulsion may be produced. 

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